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Water Resources--Office of Water Quality

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The range of ideal colony counts differs depending on the fecal indicator group to be enumerated (table 7.1-9). Crowding and insufficient media to support full development of colonies can result if the bacterial concentration on the filter exceeds the upper limit of the ideal range. The lower limit of the ideal range is set as a number below which statistical validity becomes questionable (Britton and Greeson, 1989, p. 14). For potable waters, results are routinely reported from counts of less than 20 colonies per filter. The computation is the sum of the colony counts for each sample volume, multiplied by 100, and divided by the sum of the sample volumes.

arrow Enumeration results for the membrane-filter method are expressed as a concentration in units of col/100 mL.

arrow Whole numbers are reported for results less than 10, and two significant figures are reported for results greater than or equal to 10.

The examples that follow include most of the colony-count situations commonly experienced:

Case 1

Case 2

Case 4

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